Difference Between Annual Precipitation & Evaporation Potential

Definitions

• Annual precipitation is an estimate of the amount of precipitation that reaches the ground in any given region in a calendar year. Precipitation is water in form, whether as a liquid (rain), a solid (snow or ice) or a combination of liquid and solid (freezing rain/sleet/slush). Evaporation potential, also called potential evapotranspiration (PET), measures the conditions available to allow the processes of evaporation (water turning to vapor in the atmosphere) and transpiration (plants releasing water vapor).

Quantifiable Measurement

• Annual precipitation and evaporation potential both deal with quantities of water in the atmosphere. However, annual precipitation is a very concrete estimation and evaporation potential is not. Although it is impossible for humans to measure exactly how much precipitation falls to the earth in any area, weather researchers and meteorologists make very educated guesses based on several factors, including the temperature of the region, humidity, wind patterns and ocean currents. Evaporation potential uses several complex equations -- such as Eddy covariance, which measures vertical wind speed in an area in relation to water vapor density -- to determine how much water vapor potentially up to the sky. But these measurements are not as precise as measurements used to determine annual precipitation

Stages of the Water Cycle

• Annual precipitation and evaporation potential deal with two different stages of the water cycle. The water cycle is the natural system through which water falls to the Earth and is taken back up into the atmosphere, where it falls to Earth again. Annual precipitation measures only what actually reaches the ground. A lot of precipitation never reaches the ground, but rather evaporates before hitting Earth. Evaporation potential focuses on the transfer of water vapor from the ground back to the atmosphere.

Instrumentation

• Several scientific instruments are used by meteorologists to measure and estimate annual precipitation, including barometers and weather radar systems. No instrument is used to specifically measure evaporation potential, although an experimental device called the lysimeter measures the water in a soil sample to analyze the rate of water retention.